In order to meet the growing demand for petroleum products there is greater utilization of sour crudes, which when combined with tighter environmental legislation regarding the concentration of nitrogen and sulfur within fuel, leads to accentuated refining problems. The removal of sulfur (hydrodesulfurization—HDS) and nitrogen (hydrodenitrification—HDN) containing compounds from fuel feed stocks is targeted during the hydrotreating steps of refining and is achieved by the conversion of organic nitrogen and sulfur to ammonia and hydrogen sulfide respectively.
Since the late 1940s the use of catalysts containing nickel (Ni) and molybdenum (Mo) or tungsten (W) have demonstrated up to 80% sulfur removal. See for example, V. N. Ipatieff, G. S. Monroe, R. E. Schaad, Division of Petroleum Chemistry, 115th Meeting ACS, San Francisco, 1949. For several decades now there has been an intense interest directed towards the development of materials to catalyze the deep desulfurization, in order to reduce the sulfur concentration to the ppm level. Some recent breakthroughs have focused on the development and application of more active and stable catalysts targeting the production of feeds for ultra low sulfur fuels. Several studies have demonstrated improved HDS and HDN activities through elimination of the support such as, for example, Al2O3. Using bulk unsupported materials provides a route to increase the active phase loading in the reactor as well as providing alternative chemistry to target these catalysts.
More recent research in this area has focused on the ultra deep desulfurization properties achieved by a Ni—Mo/W unsupported ‘trimetallic’ material reported in, for example, U.S. Pat. No. 6,156,695. The controlled synthesis of a broadly amorphous mixed metal oxide consisting of molybdenum, tungsten and nickel, significantly outperformed conventional hydrotreating catalysts. The structural chemistry of the tri-metallic mixed metal oxide material was likened to the hydrotalcite family of materials, referring to literature articles detailing the synthesis and characterization of a layered nickel molybdate material, stating that the partial substitution of molybdenum with tungsten leads to the production of a broadly amorphous phase which, upon decomposition by sulfidation, gives rise to superior hydrotreating activities.
The chemistry of these layered hydrotalcite-like materials was first reported by H. Pezerat, contribution à l'étude des molybdates hydrates de zinc, cobalt et nickel, C. R. Acad. Sci., 261, 5490, who identified a series of phases having ideal formulas MMoO4.H2O, EHM2O− (MoO4)2.H2O, and E2-x(H3O)xM2O(MoO4)2 where E can be NH4+, Na+ or K+ and M can be Zn2+, Co2+ or Ni2+.
Pezerat assigned the different phases he observed as being Φc, Φx or Φy and determined the crystal structures for Φx and Φy, however owing to a combination of the small crystallite size, limited crystallographic capabilities and complex nature of the material, there were doubts raised as to the quality of the structural assessment of the materials. During the mid 1970s, Clearfield et al attempted a more detailed analysis of the Φx and Φy phases, see examples A. Clearfield, M. J. Sims, R. Gopal, Inorg. Chem., 15, 335; A. Clearfield, R. Gopal, C. H. Saldarriaga-Molina, Inorg. Chem., 16, 628. Single crystal studies on the product from a hydrothermal approach allowed confirmation of the Φx structure, however they failed in their attempts to synthesize Φy and instead synthesized an alternative phase, Na—Cu(OH)(MoO4), see A. Clearfield, A. Moini, P. R. Rudolf, Inorg. Chem., 24, 4606.
The structure of Φy was not confirmed until 1996 by Ying et al. Their investigation into a room temperature chimie douce synthesis technique in the pursuit of a layered ammonium zinc molybdate led to a metastable aluminum-substituted zincite phase, prepared by the calcination of Zn/Al layered double hydroxide (Zn4Al2(OH)12CO3.zH2O). See example D. Levin, S. L. Soled, J. Y. Ying, Inorg. Chem., 1996, 35, 4191-4197. This material was reacted with a solution of ammonium heptamolybdate at room temperature to produce a highly crystalline compound, the structure of which could not be determined through conventional ab-initio methods. The material was indexed, yielding crystallographic parameters which were the same as that of an ammonium nickel molybdate, reported by Astier, see example M. P. Astier, G. Dji, S. Teichner, J. Ann. Chim. (Paris), 1987, 12, 337, a material belonging to a family of ammonium-amine-nickel-molybdenum oxides closely related to Pezerat's materials. Astier did not publish any detailed structural data on this family of materials, leading to Ying et al reproducing the material to be analyzed by high resolution powder diffraction in order to elucidate the structure. Ying et al named this class of materials ‘layered transition-metal molybdates’ or LTMs.
Since the initial reports of unsupported Ni—Mo/W oxidic precursors, U.S. Pat. No. 6,156,695, there have been several reports describing materials which, when sulfided, claim to have enhanced hydrotreating activities. U.S. Pat. No. 6,534,437 discloses a process for preparing a mixed metal catalyst having a powder x-ray diffraction pattern showing reflections at approximately 2.53 Angstroms and 1.70 angstroms. U.S. Pat. No. 6,534,437 differentiates itself from U.S. Pat. No. 3,678,124 and WO 9903578 based on characteristic full width half maximum line widths of more resolved reflections, present in WO 9903578, claiming that the invention of U.S. Pat. No. 6,534,437 possesses a ‘different microstructure’ from prior work, WO 9903578.
U.S. Pat. No. 8,722,563 describes preparing a series of catalyst precursors with compositions comprising at least one Group VI metal and one metal from Group VIII through Group X. One of the comparative examples described in the patent yields a comparable powder x-ray diffraction pattern to that obtained in U.S. Pat. No. 6,534,437 and is described as the as-synthesized and dried hexagonal NiWO4 catalyst precursor.
U.S. Pat. No. 7,648,941 discloses synthetic examples of a series of different bimetallic materials and states that the bulk bimetallic catalyst of the invention has a metastable structure and further assert that the crystalline 2θ structure of the metastable hexagonal NiWO4 phase in the preferred catalysts of the invention have lattice parameters a=2.92 Å, b=2.93 Å, and c=4.64 Å and that bulk catalyst has a metastable hexagonal structure having an X-ray diffraction pattern with a single reflection between 58 and 65°. As a point of reference, the largest two d-spacings which can be generated in an x-ray diffraction pattern by a hexagonal cell with lattice parameters a=2.92 Å, b=2.93 Å, and c=4.64 Å are 4.64 Å and 2.53 Å.
A. Dias and V. S. T. Ciminelli, J. Eur. Ceramic. Soc, 2001, 21, 2061-2065 reported on the thermodynamic calculations and modeling of hydrothermally synthesized nickel tungstates. They present a series of calculated yield diagrams at various synthesis temperatures highlighting the pH and reagent concentrations which yield NiWO4. All of their calculations predict the formation of a nickel tungstate between pH 2 and 7.5, with nickel hydroxide being the main product at higher pH's. The authors show the x-ray diffraction patterns for the samples produced at 200, 230 and 260° C. within and without their predicted yield zones. The x-ray diffraction pattern for the NiWO4 material synthesized at 200° C. can be described as poorly crystalline and the reference asserts that it is important to note that a crystallized material was obtained at 200° C., but with extremely fine particle size indicated by broad X-ray diffraction peaks. The reference asserts this can be explained by the energy barrier for the precipitation, which is closely related to the nature of the rate-controlling step in the dominant formation process. The reference puts forth that higher reaction temperatures accelerate the crystallization process because of greater thermal energy to overcome the energy barrier for transformation, and a consequence, materials with higher crystallinity and/or particle size can be obtained. The reference suggests that the phase obtained at 200° C. is essentially a poorly crystalline, nano-wolframite (NiWO4), and this conclusion is consistent with calculated yield diagrams of the reference.
Y. Bi, H. Nie, D. Li, S. Zeng, Q. Yang and M. Li, Chemical Communications, 2010, 46, 7430-7432 discuss the preparation of NiWO4 nanoparticles as a promising hydrodesulfurization catalyst, stating that all the reflections in a typical powder x-ray diffraction pattern can be indexed undisputedly to the monoclinic NiWO4, Wolframite, phase. The reference asserts that FIG. 1 shows the typical X-ray diffraction (XRD) pattern of the as-made sample and all reflections can be indexed undisputedly to the monoclinic NiWO4 phase (JCPDS card 72-1189). The reference concludes that a close examination reveals that the reflections in the XRD pattern are a little broad, indicating the characteristic feature of nanosized materials.